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"Clamp" is a minimal framework for command-line utilities.

It handles boring stuff like parsing the command-line, and generating help, so you can get on with making your command actually do stuff.

Not another one!

Yeah, sorry. There are a bunch of existing command-line parsing libraries out there, and Clamp draws inspiration from a variety of sources, including Thor, optparse, and Clip. In the end, though, I wanted a slightly rounder wheel.

Quick Start

Clamp models a command as a Ruby class; a subclass of Clamp::Command. They look something like this:

class SpeakCommand < Clamp::Command

  option "--loud", :flag, "say it loud"
  option ["-n", "--iterations"], "N", "say it N times", :default => 1 do |s|

  parameter "WORDS ...", "the thing to say"

  def execute

    signal_usage_error "I have nothing to say" if arguments.empty?
    the_truth = arguments.join(" ")
    the_truth.upcase! if loud?

    iterations.times do
      puts the_truth



Class-level methods (like option and parameter) are available to declare command-line options, and document usage.

The command can be invoked by instantiating the class, and asking it to run:"speak").run(["--loud", "a", "b", "c"])

but it's more typical to use the class-level "run" method:

which takes arguments from ARGV, and includes some handy error-handling.

Declaring options

Options are declared using the option method. The three required arguments are:

  1. the option switch (or switches),
  2. a short description of the option argument type, and
  3. a description of the option itself

For example:

option "--flavour", "FLAVOUR", "ice-cream flavour"

It works a little like attr_accessor, defining reader and writer methods on the command class. The attribute name is derived from the switch (in this case, "flavour"). When you pass options to your command, Clamp will populate the attributes, which are then available for use in your #execute method.

def execute
  puts "You chose #{flavour}.  Excellent choice!"

If you don't like the inferred attribute name, you can override it:

option "--type", "TYPE", "type of widget", :attribute_name => :widget_type
                                           # to avoid clobbering Object#type

Short/long option switches

The first argument to option can be an array, rather than a single string, in which case all the switches are treated as aliases:

option ["-s", "--subject"], "SUBJECT", "email subject line"

Flag options

Some options are just boolean flags. Pass ":flag" as the second parameter to tell Clamp not to expect an option argument:

option "--verbose", :flag, "be chatty"

For flag options, Clamp appends "?" to the generated reader method; ie. you get a method called "verbose?", rather than just "verbose".

Negatable flags are easy to generate, too:

option "--[no-]force", :flag, "be forceful (or not)"

Clamp will handle both "--force" and "--no-force" options, setting the value of "#force?" appropriately.

Validation and conversion of option arguments

If a block is passed to option, it will be called with the raw string option argument, and is expected to coerce that String to the correct type, e.g.

option "--port", "PORT", "port to listen on" do |s|

If the block raises an ArgumentError, Clamp will catch it, and report that the option value was bad:

ERROR: option '--port': invalid value for Integer: "blah"

Declaring parameters

The parameter method is used to declare positional command parameters:

parameter "FILE ...", "source files"
parameter "DIR", "target directory"

Use of parameter is entirely for documentation purposes. Whether or not you declare and describe your expected arguments, the actual arguments that remain after option parsing will be available as arguments when your #execute method is called.


The subcommand method declares sub-commands:

class MainCommand < Clamp::Command

  subcommand "init", "Initialize the repository" do

    def execute
      # ...



Clamp generates an anonymous sub-class of the current class, to represent the sub-command. Additional options may be declared within subcommand blocks, but all options declared on the parent class are also accepted.

Alternatively, you can provide an explicit sub-command class, rather than a block:

class MainCommand < Clamp::Command

  subcommand "init", "Initialize the repository", InitCommand


class InitCommand < Clamp::Command

  def execute
    # ...


When a command has sub-commands, Clamp will attempt to delegate based on the first command-line argument, before options are parsed. Remaining arguments will be passed on to the sub-command.

Getting help

All Clamp commands support a "--help" option, which outputs brief usage documentation, based on those seemingly useless extra parameters that you had to pass to option and parameter.

$ speak --help
    speak [OPTIONS] WORDS ...

    WORDS ...                     the thing to say

    --loud                        say it loud
    -n, --iterations N            say it N times
    --help                        print help

Contributing to Clamp

Source-code for Clamp is on Github.

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